From Slate:

This year’s Nobel Prize in chemistry has been awarded to three scientists whose work has greatly improved what we can look at under a microscope. Things that were once thought too small for optic magnification—such as the structures of proteins, the movement of viruses—can now be observed by the human eye and captured through the lens of a camera.

Light (or optic) microscopes use a light source and a system of lenses to bring the miniature into view. But they are constrained by natural properties of optics: Image resolution is limited by the wavelength of light used—anything smaller than about 0.2 millionths of a meter (or 200 nanometers) could not appear any clearer than a blob. (A different kind of microscope, called an electron microscope, does not have this size restriction, but it can only look at specially fixed samples, not living cells or organisms.)

Read more.